In the manufacture of tissue-based products such as facial and bath tissue, paper towels, and napkins, the wet tissue web is commonly dewatered and then dried on one or more through-air-dryers (TADs.) A TAD is an open-deck cylinder that supports a throughdrying fabric, which in turn supports the wet tissue web being made. This method employs passing heated air from a hood, through the wet tissue web and fabric, and into the open TAD. The hot air is cooled as it moves through the wet tissue web and picks up moisture. Some of the air is exhausted to decrease the moisture build-up within the TAD system and the remainder of the air is then recycled to a burner where fresh makeup air may be introduced. The air is then reheated and returned through the wet web to the TAD to complete the cycle.
The throughdrying technique is advantageous in that it allows high-bulk sheets to be made by molding the paper web onto a highly topographic fabric as it is passed over the TADs. Because the motive force used to mold and dry the web is hot, relatively dry air, the capital and energy costs of a TAD system can be quite expensive in comparison to the costs for a standard wet-pressed tissue machine. During the drying process in general, and throughdrying in particular, the energy efficiency is high in the initial stages of drying, but tends to become progressively lower as water is removed from the tissue web. Generally, this reduced efficiency must be accepted when drying is being carried out in the falling-rate drying zone where mass-transfer-limited drying becomes dominant.
In general, the final moisture content of a tissue web, and paper universally, is roughly 5%. Expressed in terms of consistency, the final, or reel, basesheet consistency is about 95%. This final moisture content is roughly the equilibrium moisture content of tissue or paper exposed to air. Thus, the tissue web or paper at ambient humidity will contain roughly 5% moisture, though most would consider it to be xe2x80x9cdry.xe2x80x9d Hence there is little incentive for the tissue maker to dry the tissue web to less than 5% final moisture content as the tissue web will re-absorb moisture from the ambient air and re-equilibrate at the 5% moisture content level.
Given the high cost of drying in the low moisture regime, the tissue manufacturer strives to manufacture product at the highest possible final moisture. Although the additional amount of water removed is very small, drying a tissue web to about 3% moisture may require an additional 10% more energy than drying a tissue web to about 5% moisture. For example, in a standard throughdried tissue-making process where the wet tissue web enters the throughdryers at about 33% consistency (about 2 pounds of water per pound of fiber), the additional water removal from the 5% moisture content to the 3% moisture content (only 0.02 pounds of water per pound of fiber) represents about 1% of the total drying load. It is not surprising the tissue maker is reluctant to spend approximately 10% more energy to remove only 1% more water, especially when this is normally not required to improve product quality.
The only incentive for additional water removal would be if the improvement in product properties associated with the additional water removal would exceed the cost of the additional drying. However, in most paper processes, adequate properties can be achieved at a final moisture content level of 5%. Any additional drying would not add value, and hence is avoided.
However, in some tissue making processes, especially those where the creping step has been eliminated, as in uncreped through-air dried (UCTAD) technology, the final tissue web moisture content is a major determinant of the product properties, and in these cases, it is necessary to have a very low moisture content at the reel of the tissue machine. For example, the uncreped throughdried tissue process described in U.S. Pat. No. 5,607,551 issued on Sep. 30, 1997 to Farrington et al. requires that the moisture content of the tissue web be reduced to approximately 1% moisture in order to maximize sheet softness. In this and other related processes, it is imperative that the final sheet moisture be as low as possible in order to maintain the softness of the tissue web through any calendering operations. Hence, in such processes, it is highly desirable to develop an efficient drying process for drying in the very low moisture regime of about 5% moisture to about 1% moisture.
Similarly, for wet-pressed tissue, improved product properties can be achieved by drying the sheet to very low moistures followed by creping. Final moistures may be as low as 1% to 3%. Again, a high-efficiency drying process for moistures below 5% is highly desirable.
To explain more fully the mechanism of drying paper or tissue, an understanding of the states of water in cellulosic webs is useful. In cellulosic fibers, three forms of water are present. Bulk water is present within the fiber cell in macropores, the areas that remain when lignin and hemicellulose are removed during the pulping process. Freezing bound water is present in the amorphous areas of the fiber""s lamellae. The final category of water is non-freezing bound water, which is adsorbed onto hydrophilic groups in the cell wall, such as hydroxyl groups. As moisture is removed and the wet tissue web is dried, two significant moisture transitions are crossed. At a moisture ratio of between about 0.5 to about 0.8 pound water per pound fiber, all of the bulk water has been removed from the fiber cell, mostly by mechanical means, and all remaining water is present in the form of freezing or nonfreezing bound water. Beginning at a moisture ratio of about 0.25 pounds water per pound fiber, the pores of the fiber collapse and only non-freezing water that is bound to the hydroxyl groups remains. This water requires high amounts of energy to remove. It is in this region that an auxiliary drying method becomes most important. Such auxiliary drying may be accomplished using infrared dryers, microwave dryers, radio frequency dryers, sonic dryers, dielectric dryers, ultraviolet dryers, and combinations thereof.
It has been unexpectedly discovered that drying the tissue web with an auxiliary dryer from about 5% to about 1% moisture requires an order of magnitude less energy per pound of water removed from the tissue web vs. a drying process using only conventional means (primary dryers), such as a TAD system, a Yankee dryer system, or Yankee dryer/hood combination system. The primary dryer could also be a condebelt apparatus or high-intensity nip press dryer. The efficiency of the auxiliary drying in the low moisture regime is especially apparent when evaluated against current practices. For example, compared to a 50,000 BTU per pound water requirement by both a commercial and a pilot throughdryer system to dry a tissue web from about 0.03 to about 0.01 pounds of water per pound of fiber moisture content range, the auxiliary dryer, such as a microwave dryer, required only about 4,000 to about 8,000 BTU per pound water removed. This increase in drying efficiency can translate to a machine speed increase during the drying process to achieve a given level of dryness or an increased level of dryness at current machine speeds or even an energy savings at constant level of dryness and machine speed. It would be particularly advantageous to situate an auxiliary dryer after the last primary dryer, such as a throughdryer, to remove the last few percent moisture in the tissue web. This would allow the primary dryers, like throughdryers, to operate at a lower temperature or load because of the increased final level of moisture in the tissue web required as the tissue web exits the primary dryer and enters the auxiliary dryer.
Hence, in one aspect, the present invention resides in a process for making tissue comprising: (a) forming the wet tissue web by depositing an aqueous suspension of papermaking fibers onto a forming fabric; (b) partially dewatering the wet tissue web; (c) partially drying the wet tissue web in at least one primary dryer; (d) additionally drying the wet tissue web further by passing the wet tissue web through an auxiliary dryer, wherein the auxiliary dryer dries the wet tissue web to a final moisture content of about 5% or less, thereby forming a dried tissue web; and (e) winding the dried tissue web into a parent roll.
In another aspect, the present invention resides in a process for making tissue comprising: (a) forming the wet tissue web by depositing an aqueous suspension of papermaking fibers onto a forming fabric; (b) partially dewatering the wet tissue web; (c) partially drying the wet tissue web in at least one throughdryer; (d) additionally drying the wet tissue web further by passing the wet tissue web through an auxiliary dryer, wherein the auxiliary dryer dries the wet tissue web to a final moisture content of about 5% or less, thereby forming a dried tissue web; and (e) winding the dried tissue web into a parent roll.
In another aspect of the present invention, an auxiliary dryer is placed between two primary dryers, thereby adjusting the moisture profile of the wet tissue web prior to final drying. As discussed below, the moisture content of the wet tissue web is not evenly distributed throughout the web, causing preferential and inefficient drying of the wet tissue web. Use of the auxiliary dryer can provide a more uniform moisture profile by preferentially drying the wet areas of the tissue web, thereby allowing for more efficient drying as the wet tissue web is passed over the following primary dryer. In addition, less drying may be required if the areas of the wet tissue web having higher than average moisture were preferentially dried, providing a more uniform moisture profile of the wet tissue web, thereby allowing for more efficient drying as the wet tissue web is passed over the following primary dryer.
According to another aspect of the present invention, an auxiliary dryer is placed between two throughdryers, thereby adjusting the moisture profile of the wet tissue web prior to final drying. As discussed below, the moisture content of the wet tissue web is not evenly distributed throughout the web, causing preferential and inefficient drying of the wet tissue web. Use of the auxiliary dryer can provide a more uniform moisture profile by preferentially drying the wet areas of the tissue web, thereby allowing for more efficient drying as the wet tissue web is passed over the following throughdryer. In addition, less drying may be required if the areas of the wet tissue web having higher than average moisture were preferentially dried, providing a more uniform moisture profile of the wet tissue web, thereby allowing for more efficient drying as the wet tissue web is passed over the following throughdryer.
Other aspects of the present invention will be apparent in view of the following description of the preferred embodiments and the accompanying drawings.